FI91469C - Method of tuning an RF bandpass filter - Google Patents

Abstract

PCT No. PCT/FI93/00128 Sec. 371 Date Nov. 30, 1993 Sec. 102(e) Date Nov. 30, 1993 PCT Filed Mar. 30, 1993 PCT Pub. No. WO93/20616 PCT Pub. Date Oct. 14, 1993.A method for tuning an RF bandpass filter, especially a combiner filter (101, 102. . . 10n) belonging to a GSM system, in which an RF signal having a certain nominal carrier frequency (fc) is inputted into the RF bandpass filter and a medium frequency of a pass band of the RF bandpass filter is tuned depending on an RF power propagating through the bandpass filter or on an RF power reflected from an input of the bandpass filter. For an easier tuning of the filter and for a tuning accuracy better than before, (a) a carrier is modulated by a signal causing a first predetermined offset (+ DELTA f), of the carrier frequency, (b) a medium frequency, at which the power propagating through the filter is at the maximum or the power reflected from the input of the filter is at the minimum, is searched for for the bandpass filter, (c) the first medium frequency obtained at stage (b) is stored, (d) the carrier is modulated by a signal causing a second predetermined offset (- DELTA f) of the carrier frequency, (e) a medium frequency, at which the power propagating through the filter is at the maximum or the power reflected from the input of the filter is at the minimum, is searched for for the bandpass filter, (f) the second medium frequency obtained at stage (e) is stored, and (g) the filter is tuned by tuning its medium frequency to a value determined on the basis of the frequency values obtained at stages (c) and (f).

Description

i 91469

The invention relates to methods for tuning an RF bandpass filter according to the preambles of the appended claims 1 and 3.

The methods according to the invention are intended to be used in particular in the so-called GSM or similar mobile telephone-10 network. in the tuning of combiner filters, but they can also be applied to bandpass filters suitable for other purposes, where frequency-selective power measurement is required for frequency control.

A combiner is a device that connects multiple transmitters 15 to the same antenna or antenna line. In this case, each radio transmitter is connected to its own bandpass filter, the so-called through a combiner filter to the antenna or antenna line. The center frequency of each bandpass filter is tuned to the center frequency of the corresponding radio transmitter. The purpose of the Suo-20 data is, on the one hand, to feed the transmission signal to the smallest hSvid antenna in its own radio transmitter and, on the other hand, to prevent the transmission of signals of different frequencies from other radio transmitters from the antenna direction to its own radio transmitter. Traditionally, combiner filters are fixedly tuned to the transmission frequencies of radio transmitters. In this case, the transmission frequency of the radio transmitter could not be changed without changing the combiner filter or its tuning.

However, it is often desirable to be able to change the frequencies of ra-30 diol transmitters simply and quickly. Such a case is, for example, a base station of a cellular mobile telephone system to which certain transmission and reception channels have been assigned. If the channel deficiency of the system can be changed, if desired, by changing the transmission and reception frequencies of the base stations, it is possible to make flexible and efficient use of the channel capacity of the system in changing conditions. For this reason, combiner filters have been developed whose center frequency changes automatically as the transmission frequency changes.

5 The setting of the automatically tuned combiner filters is based on the measurement of the RF power reflected from the input of the filters or the RF power passing through the combiner filter. The center frequency of the filter is locked to the frequency at which the reflected power is at a minimum or the forward power is at a maximum.

In GSM and PCN mobile telephone networks, the so-called transmitter signal modulation method is used. GMSK (Gaussian Minimum Shift Keying) modulation. This method of modulation is relatively wideband with signals corresponding to the operating conditions, which makes it difficult to tune the combiner filters to the correct frequency, and the output accuracy obtained is rather modest.

The object of the present invention is therefore to provide a method by means of which the combiner filters can be tuned even more precisely to the correct frequency. This is achieved by a method according to a first embodiment of the invention, characterized by what is described in the characterizing part of appended claim 1, or by a method according to a second embodiment of the invention, characterized by what is described in the characterizing part of appended claim 3.

The basic idea according to the invention is to find for the bandpass filter the center frequency at which the power passing through the filter is at its maximum or the power reflected from the inlet of the filter at least (at least) twice using the modulus to a predetermined frequency, and determine the final frequency.35 Based on (at least) two excitation values obtained from 91469 3 tunings performed on these RF signals.

With the solution according to the invention, it is possible to tune the RF band fast filter more easily and accurately to the correct frequency.

In the following, the invention and its preferred embodiments will be explained in more detail with reference to the examples according to the accompanying drawings, in which Fig. 1 shows a block diagram of a typical composite filter system 2 shows a block diagram of a typical transmitter system implemented with comp 15-filter filters, in which the center frequency of the fast band of a bandpass filter (combiner filter) is obtained from the input of the filter in terms of the reflected RF power. in transmitters, Fig. 4a shows a typical GMSK modulation spectrum, Fig. 4b shows the spectrum of the first signal used in the method 25 according to the invention, and Fig. 4c shows the spectrum of the first signal used in the method according to the invention. a spectrum of the second signal to be used.

Figure 1 shows a radio transmitter system belonging to a GSM or similar mobile telephone network, which comprises n .30 radio transmitters TX1, TX2 ... TXn, respectively, having transmission frequencies f1, f2 ... fn, for example in the area 920 -960 MHz. Each radio transmitter is connected to band-fast filters I0lf 102 tuned to the corresponding frequencies. ·. 10n to a common summing-55 point SI and further via a directional switch member 12 to 4 common transmitting antennas ANT. (In this example, the switching of each transmitter to its band-pass filter takes place via the rotating means 112, 112 and 11, respectively. However, the rotating means are not indispensable to the invention.) The signal supplied to the transmitting antenna ANT thus includes the frequencies of all transmitters. The bandpass filters 102, 102 ... 10n shown in Figure 1, which connect a plurality of transmitters to a common antenna, are commonly referred to as combiner filters.

The radio transmitter system further provides a control unit 13 which controls a stepper motor 14lf 142 and 14n, respectively, associated with each combiner filter, each of which encapsulates the center frequency of its own combiner filter. Each filter is a narrowband filter, the center frequency of which is sought to be as close as possible to the frequency of the transmission carrier, so that the signal to be transmitted is transmitted to the antenna ANT with the lowest possible frequency. In the example of Fig. 1, the center frequency is obtained in a manner known per se depending on the advancing RF-20 power of the bandpass filter 1Spi. The direction switch 12 is taken separately from the output signal of each filter as a sample signal PF, which is proportional to the to the past transmission frequency signal power of the filter. The sample signal is applied to a control unit 13 which, by means of a stepper motor 25, transmits the center frequency of the filter so that the level of the nMyte signal is at its maximum.

The example according to Fig. 2 otherwise corresponds to the embodiment of Fig. 1, but now the tuning of each combiner filter 102 ♦ · 10n takes place in another known manner 30, i.e. depending on the RF power reflected back from the input of the combiner filter. For this reason, there is no directional switch in the antenna line, but a separate directional switch member 122, 122 and 12n, respectively, are connected to each transmitter branch, the input of the combiner filter. In this case 35, for each directional switch noise, a signal PR is proportional to the power reflected from the input of the filter 91469 5, proportional to its power, which is applied to the control unit 13. The control unit 13

The two known methods of tuning the combiner filter on which the method according to the invention is based have been described above. The method steps according to the invention are explained in more detail below.

Fig. 3 shows a block diagram of a GMSK modulator belonging to the transmitters TX1 ... TXn, which is known per se, and Fig. 4a shows a typical signal spectrum S1 obtained as a result of GMSK modulation. The GMSK modulator provides a coding unit 31, a Gaussian filter 32 to which the output of the 15 coding units is connected, and a modulation unit 33 to which the output of the filter is connected. The encoding unit performs differential encoding of the incoming data, the encoded signal is filtered in a Gaussian filter 32, and the filtered signal is modulated to a carrier frequency in the modulation unit 33. Since the structure of the GMSK modulator is known per se, it is not related to the actual inventive idea. weather. enempSa. More specifically, the operation of the modulator is reflected, for example, in the GSM specification 05.04. (version 3.1.1) from sections 2.4-2.6.

The spectrum S1 of the RF frequency signal output from the output of the modulation unit 33 and input to the computer filter is typically as shown in Figure 4a, i.e. relatively wide. The modulating data of the field has been data to be transmitted in use or random data, which resembles data to be transmitted in use.

However, GMSK modulation has the property that the spectrum of the signal resulting from the modulation results in a terSva peak (carrier) for a frequency different from the actual nominal carrier frequency fc (fc 6 is fl, f2, etc. or fn) when A specific predefined bit string is used as the smooth data. In other words, the carrier appears as a sharp peak at a frequency that has shifted by a predetermined 5 seconds from the nominal carrier frequency. As an example, the used bit string 11111 ... shifts the indicated spectrum peak from the nominal carrier frequency by + (1 / 4T) when 1 / T is the available transmission rate (in the GSM system, the transmission rate is 270.833 kbit / s). When the corresponding bit string 010101 ... is used, said spectral peak shifts from the nominal carrier frequency by - (1 / 4T).

According to the invention, this feature is exploited in the tuning of the combiner filters by first modulating the carrier, e.g. by a bit string 1111 .., which is shown in Fig. 3 by the reference symbol A1. In this case, the spectrum S2 of the RF frequency received from the modulator filter 33 at the input of the combiner filter is as shown in Fig. 4b. The spectrum now has a terSvS peak at 20 (carrier) at a frequency that has shifted by + Af (Δί = 1/4 sivu) aside from the nominal carrier frequency fc. Using the TStS signal, the center frequency at which the power passing through the filter is at its maximum or the power 25 reflected from the input of the filter is at a minimum is applied to the combiner filter, i.e. the method described in connection with either Figure 1 or 2 is used. The first center frequency information obtained by the NS is stored in memory.

In the next step, the carrier is modulated, e.g. with the bit string 010101 ..., which is denoted in Fig. 3 by reference numeral A2. The spectrum S3 of the RF frequency input from the modulator unit to the input of the combiner filter is then as shown in Fig. 4c. SpektrissM is now a sharp peak (carrier) at a frequency shifted by -Af (Af = 1 / 4T) aside from the nominal carrier-35 true frequency fc. (The spectra S1 and S2 resulting from the modulation result included in the use are weaker components at other frequencies as well, but for the sake of clarity they are not shown in Figures 4b and 4c.) is at its maximum or the power reflected from the filter inside the filter is at a minimum, i.e. the method described in connection with either Figure 1 or 2 is reused. The second center frequency information thus obtained is stored in memory.

As a final step, the final tuning of the combiner filter is performed by tuning the center frequency to halfway between the first and second center frequency information.

Although the invention has been described above with reference to the example according to the accompanying drawings, it is clear that the invention is not limited thereto, but can be modified in many ways within the scope of the inventive idea set forth above and in the appended claims.

Thus, although the invention has been described with reference to, for example, a combiner filter belonging to the GSM system, the invention is applicable to other systems using GMSK modulation or to other modulation methods having similar properties. In principle, the method can also be implemented in such a way that the first and second shifts of the carrier frequency have different absolute values. In the example described above, tSmå means that a different baud rate is used in the corresponding process steps.

The methods can be combined in such a way that both the reflected RF power and the flattened RF-30 power measurement are used in the output, so that the control unit selects the respective delivery method depending on the external load.

Since different load situations affect the sample signals Pp and PR in different ways, the effect of these external loads on the crop can be reduced by the combined escrow mode.

Claims (4)

8 A method for tuning an RF bandpass filter, in particular a composite filter (10lr 102 ... 10n) belonging to a GSM or similar system, in which a RF signal of a nominal frequency is input to the RF bandpass filter. -frequency (fc), and the center frequency of the fasting band of the RF bandpass filter is obtained depending on the RF power passing through the bandpass filter 10, characterized by the steps of (a) modulating the carrier applied to the filter (10lf 102 ... 10n) (b) retrieving for the bandpass filter the center frequency at which the propagating power of the filter lSpi is at its maximum, (c) storing the first center frequency obtained in step (b), (d) the carrier applied to the filter (10lf 102 ... 10n) is modulated with a signal which causes the carrier frequencies a second predetermined offset (-Af) from its nominal frequency (fc), (e) retrieving to the bandpass filter the center frequency at which the power passing through the filter is at its maximum, (f) storing the second center frequency obtained in step (e), and (g) tuning the filter to obtain its center frequency based on the frequency values obtained in steps (c) and (f) to a value determined by 3.. Method according to claim 1, characterized in that in steps (a) and (d) the carrier is modulated with signals which cause substantially equal but opposite displacements to the carrier frequency, wherein in step (g) the filter is tuned to half the frequency values obtained 91469 9. . A method for tuning an RF band pMM filter, in particular a combiner filter (10 ^ 102 · ..10η), in which a RF frequency signal having a certain nominal carrier frequency (fc) is applied to an RF bandpass filter and an RF carrier. depending on the RF power reflected back from the input of the bandpM filter of the bandpass filter, depending on the RF power reflected back from the input of the bandpMM filter of the bandpass filter, characterized by the steps of modulating 10 (a) carriers with a signal causing a first -frequency at which the power reflected from the input of the filter is at a minimum, (c) storing the first center frequency obtained in step (b), (d) modulating the carrier with a signal causing a second predetermined offset (-Af) to the carrier frequency, (e) retrieving for the bandpMM the filter the center frequency at which the filter input h the reflected power is at a minimum, (f) storing the second center frequency obtained in step (e), and (g) tuning the filter sMatamMUM to its average frequency based on the frequency values obtained in steps (c) and (f). A method according to claim 3, characterized in that in steps (a) and (d) the carrier is modulated with signals which cause substantially equal but opposite displacements to the carrier frequency, wherein in step (g) the filter is tuned according to the obtained frequency values. A mid-term. 10